1. Field of the Invention
This invention relates generally to a magnetic recording medium, and more particularly is directed to an improved magnetic recording medium which is suitable for the recording of signals thereon by so-called perpendicular magnetization.
2. Description of the Prior Art
In conventional magnetic recording, for example, as employed in magnetic tape or sheet recorders and the like, the audio, video or other signals are recorded on a magnetic recording layer formed on a non-magnetic base by magnetizing such layer mainly in a longitudinal direction which is parallel to the direction of relative movement of the magnetic tape or sheet in respect to a ring-type magnetic transducer or head used for recording. When recording in such longitudinal magnetization mode, increasing the recording density, that is, the density of signals recorded on the tape or sheet, correspondingly reduces the wavelength of the recorded signal, with the result that the demagnetizing field in the magnetic recording layer is increased. Increasing the demagnetizing field causes attenuation and rotation of the residual magnetization, with the result that a reduced output is obtained when reproducing or playing back the recorded signals. On the contrary, if signals are recorded by magnetizing the magnetic recording layer in the direction normal to the surface thereof, that is, in the so-called perpendicular magnetization mode, decreasing the wavelength of the recorded signal, as by increasing the recording density, causes a reduction in the demagnetizing field. Therefore, it will be appreciated that, for recording a signal of short wavelength, recording in the perpendicular magnetization mode (hereinafter referred to as perpendicular recording) is more advantageous than the conventional recording in the longitudinal magnetization mode (hereinafter referred to as longitudinal recording).
Although various recording media have been proposed for use in connection with perpendicular recording, such recording media have not been effective, in practice, to achieve the full advantages of perpendicular recording. One recording medium heretofore proposed for perpendicular recording has been disclosed by the present inventors in the "Proceedings of the Seventh Annual Conference on Magnetics", Paper No. 4pA-7, 1975, Japan, in the "Proceedings of the 1976 Annual Meeting of the Institute of Electronics and Communication Engineers of Japan", Paper No. 184, 1976, Japan, and also in Japanese Patent Application No. 51574/1976 (which is also identified as Published Unexamined Application No. 134,706/77). Such magnetic recording medium intended for use in perpendicular recording has a magnetic recording layer with a direction of easy magnetization perpendicular to the surface of the recording layer, that is, in the direction of the thickness thereof. The magnetic recording layer having a direction of easy magnetization perpendicular to the surface thereof is made of a chromium-cobalt (Cr--Co) alloy which is formed directly on a non-magnetic base or carrier by radio frequency sputtering. Although the magnetic recording layer of chromium-cobalt alloy exhibits magnetic characteristics which favor the recording of signals therein by perpendicular magnetization rather than by longitudinal magnetization, the relatively high coercive force of the chromium-cobalt alloy makes it rather difficult to record signals on the magnetic recording layer thereof until such layer has been saturated by the single-pole magnetic transducer or head used for the perpendicular recording. Therefore, relatively great electrical power is required for the recording operation.
In order to overcome the above-mentioned problems associated with a recording medium for perpendicular recording which consists only of a magnetic recording layer of chromium-cobalt alloy on a non-magnetic base, the present inventors have further proposed, for example, as disclosed in Japanese Patent Application No. 51574/1976 (also identified as Published Unexamined Patent Application No. 134706/77) and No. 106506/1976 (also identified as Published Unexamined Patent Application No. 32009/78), a magnetic recording head capable of effecting efficient perpendicular recording with a relatively low magneto-motive force, and which includes, in addition to the single-pole facing the surface of the recording medium defined by the magnetic-recording layer, an excited auxiliary magnetic pole which faces the other side or exposed surface of the non-magnetic base of the magnetic recording medium. Although such magnetic heads for perpendicular recording are effective in some circumstances, for example, when the recording medium is a magnetic tape or sheet having a thin non-magnetic base, it is difficult to employ such magnetic heads for the recording of signals on magnetic discs or the like which have a relatively thick non-magnetic base.